Numerous prior art attempts have been made to construct multiple color cathode-ray tube systems in which the color is varied by changing the screen anode voltage and hence the beam velocity and the penetration depth of the electron beam into a multilayer phosphor screen. The earliest of these attempts included a cathode-ray tube wherein the screen electrode and conductive coating on the inside of the tube envelope formed a single electrode both connected to the same high voltage source. These early attempts suffered from two major problems. First, the screen electrode and conductive coating connected together formed a large capacitance across which the high voltage had to be switched each time the color was to be changed. This large capacitance created the need for a high powered amplifier which was capable of switching the voltage across such a capacitance in a relatively short period of time, otherwise large delay times had to be tolerated if a more reasonably powered amplifier switch was used to the high voltage. Secondly, whenever the high voltage to the screen and conductive coating was switched, the deflection sensitivity of the tube also changed since the high voltage which was connected to these electrodes was the final accelerating voltage which is determinative of the deflection sensitivity of the tube.
Later attempts divided the screen electrode from the conductive coating on the inside of the tube envelope with a conductive wire mesh inserted between the two electrodes thus formed. Although the capacitance of the screen electrode, with respect to the tube cathode and ground, was reduced somewhat by the separation, the insertion of the mesh raised the capacitance again to a higher capacitance since the mesh and screen electrodes formed a parallel plate capacitor. Furthermore, such a mesh created problems when the electron beam struck the wire of the mesh.
One attempt to circumvent the problem of having to switch a high voltage across a large capacitance included a tube in which the screen electrode and envelope coating electrode were connected together but two or more separate electron guns were used, one for each of the layers of phosphor in the screen. Each of these electron guns was connected to a different voltage so that the voltage between each gun and the screen was different. One problem with such multiple gun beam penetration cathode-ray tubes was arcing between the guns as several kilovolts of voltage difference were typically required between the electron guns to achieve usable penetration depth difference. Furthermore, the potential difference between the electron guns created lensing effect in the gun region which resulted in pattern distortion problems. Also, it was necessary to impress the video signals upon a relatively high voltage compared with their normal voltage levels.